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There are two plasma contactors on the ISS Z1 truss, designed to neutralize static electrical charge of the space station.

What generates static charge on a spacecraft? If it is picking up charge from the ionisphere, shouldn't it pick up an equal number of + and - ions?

Is the static charge a hazard to the ISS itself, or it only a hazard when docking with another spacecraft? Before docking, is there a “grounding” procedure to ensure both spacecraft are at the same potential?

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You are right; objects in space collect I+ and e- simultaneously. This results in a negative charge due to the rate at which these different particles are collected. Since electrons are far lighter than ions, they have much higher speeds at the same temperature, due to the Maxwell–Boltzmann distribution. Therefore, they have a higher probability of being collected, thereby charging the object negative until its potential is high enough to repel further electrons.

With the following equation, one can calculate the potential of a spacecraft inside a space environment.

enter image description here

In general, this static charge can damage electrical components if it for example results in a discharge into an otherwise decoupled circuit. Some components are also quite sensitive to negative potentials across them.

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